Silicon carbide is thermally and chemically very stable and is excellent in heat resistance and mechanical strength, and is therefore used as an environment resistant semiconductor material. It is also known that silicon carbide has a crystalline polytype structure. Crystal polytype is a phenomenon in which crystals have a lot of various structures even in the case of the same chemical composition. Assuming that a molecule obtained by combining Si and C is as one unit in a crystal structure, the crystal polymerization in caused by the fact that a periodic structure in case of laminating this unit structural molecule in the c-axis direction ([0001] direction) of the crystal varies.
Typical crystal polytype includes 2H, 3C, 4H, 6H and 15R. Herein, a first numeral denotes a repeating period of lamination, a letter denotes a crystal system, i.e., H denotes a hexagonal system, R denotes a rhombohedral system, and C denotes a cubic system. The respective crystal structures differ in physical and electrical characteristics and application for various uses is considered utilizing the difference. For example, 4H is used as a substrate wafer of a high frequency and high voltage resistant electric device and also 6H is used as a light emitter material for blue LED because its band gap is so large as about 3 eV. 3C is a semiconductor element material capable of operating at a high speed because of high the symmetric property of a crystal and large mobility of electrons.
Incidentally, as a method for growing a silicon carbide single crystal, for example, a vapor phase growth method, an Acheson method and a solution growth method have conventionally been known.
The vapor phase growth method includes, for example, a sublimation method (improved relay method) and a chemical vapor deposition (CVD method). The sublimation method is a method comprising sublimating a silicon carbide powder as a raw material at a high temperature of 2,000° C. or higher, and supersaturating Si, Si2C and a Si2C gas on a seed crystal substrate maintained at a low temperature thereby depositing a single crystal. The CVD method is a method comprising performing epitaxially growing a silicon carbide single crystal on a heated substrate made of Si using a silane gas and a hydrocarbon-based gas, and is used to produce a silicon carbide single crystal.
The Acheson method is a method comprising heating silicic anhydride and carbon to a high temperature of 2,000° C. or higher to produce an artificial abrasive and a single crystal is produced as a by-product.
The solution method is a method comprising melting silicon in a crucible made of a material containing carbon (generally graphite) to give a melt, dissolving carbon from the crucible in the melt, crystallizing silicon carbide on a seed crystal substrate disposed at the low temperature portion, and growing the crystal.
However, it is known that various lattice defects such as a hollow penetrating defect and stacking fault called as micropipe defect are present in the single crystal produced by the above sublimation method. Furthermore, since crystal growth is closely related to polytype transition in the sublimation method, it is difficult to reconcile control of lattice defects and control of polytype transition thereby causing such a problem that crystal polytype is likely to occur.
Also, since a small amount of a raw material is supplied because a raw material is supplied in the form of a gas in the CVD method, and the silicon carbide single crystal to be produced is limited to a thin film and it is difficult to produce a bulk single crystal as a substrate material for device.
In the Acheson method, a large amount of impurities are present in a raw material and it is difficult to enhance purity, and it is also possible to obtain a large size crystal.
On the other hand, in the solution method, since fewer lattice defects exist and also crystal polytype rarely occurs, a single crystal having good crystallinity can be obtained.
A single crystal is produced by growing (laminating) a crystal in a specific direction, and in the vapor phase method such as a sublimation method, a single crystal having a property, which is different from that of a conventional single crystal, grows on the border of certain lamination, namely, transformation of crystal polytype occurs. On the other hand, although transformation of crystal polytype can be prevented in the solution method, the resulting crystal has the same crystal structure as that of a seed crystal and a silicon carbide single crystal having a derived crystal structure cannot be obtained by controlling transformation of crystal polytype regardless of the crystal structure of the seed crystal.
Incidentally, as described above, it is now considered that a 4H-silicon carbide single crystal is suitable for applications related to a device because of its large electron mobility, inhibition band width and electrolysis breakdown, small anisotropy of electrical conduction and comparatively shallow donor or acceptor level. However, a 4H-silicon carbide scarcely exists in a relay crystal (SiC crystal made by a relay method) used as a seed crystal. Also, since the relay crystal is used as a seed crystal in the solution method, it has been difficult to produce a 4H-silicon carbide seed crystal.
An object of the present invention is to solve the above problem and provide a method capable of producing a desired 4H-silicon carbide single crystal from 6H or 15R by transforming a crystal polytype using a 6H-silicon carbide single crystal or a 15R-silicon carbide single crystal as a seed crystal.